In the case of relatively large photovoltaic systems, in particular solar parks, a feed of the generated electric power directly into a medium-voltage grid, for example a 20 kilovolt (kV) grid, is generally provided. Such solar parks generally have a multiplicity of photovoltaic modules, of which in each case a plurality are connected in series to form so-called strings. Often, a plurality of the strings are interconnected in order to supply the power generated by said strings in the form of direct current to one of possibly a plurality of inverters provided. On the output side, the inverters provided are connected to a primary circuit of a medium-voltage transformer. In this case, a medium-voltage transformer can be provided for each inverter, or a plurality of inverters can be connected to a medium-voltage transformer, possibly with separate primary windings. The generated power is fed, possibly via a coupling contactor, into the medium-voltage grid via the secondary-side output(s) of the transformer.
The article “Enel's 3-MW Power Station Preliminary Design”, 10th European Photovoltaic Solar Energy Conference by C. Corvi et al. discloses a photovoltaic system of this type in which the inverters are connected directly to the medium-voltage transformers. In this case, inverters with inverter bridges which are provided with thyristors are used. The inverters are line-commutated, i.e. draw the switching voltage, also referred to as commutation voltage for the thyristors, from the power supply grid.
Owing to their relatively low degree of efficiency, nowadays inverters comprising inverter bridges provided with thyristors are now only rarely used. More customary is the use of switching transistors in the inverter bridges in order to be able to operate the inverter with pulse width modulation. In this case, usually IGBTs (Insulated Gate Bipolar Transistors) or MOSFETs (Metal-Oxide Semiconductor Field-Effect Transistors) are used as transistors. In order to provide protection against high voltages in the non-conducting direction, these transistors are generally protected by an antiparallel diode which is conducting in the non-conducting direction of the transistor and is often already integrated in the transistor. These diodes, also referred to as freewheeling diodes, form a full-wave rectifier from the grid connection to the DC link of the inverter. If an AC voltage is present at the inverter on the grid side, but there is insufficient voltage provided by the photovoltaic modules, a return current flow through the photovoltaic modules via the freewheeling diodes is established with power being withdrawn from the AC voltage grid.
In order to prevent such reverse currents caused by the design of the inverter, it is known to connect the inverters to the medium-voltage transformers in each case via a low-voltage AC contactor.
The low-voltage AC contactors are used in order to disconnect the inverter from the grid in the event of a lack of or insufficient insolation and thus to prevent a return current flow through the photovoltaic modules with a power withdrawal from the grid. Furthermore, the low-voltage AC contactors can be used in order to decouple the respective inverter selectively from the medium-voltage transformer in the event of an overcurrent or a short circuit, in the event of infringement of the required grid parameters (voltage, frequency, fed reactive power etc.) or in the event of failure of an inverter. The fact that each inverter is provided with an associated low-voltage AC contactor and a monitoring device for maintaining the grid feed parameters is involved and cost-intensive, however.
Furthermore, it is known to arrange an AC disconnecting element between a medium-voltage transformer and the energy supply grid, which AC disconnecting element acts as an AC-side protection element. For example, the article “Electrical Fault Protection for a Large Photovoltaic Power Plant Inverter”, D. E. Collier and T. S. Key, Photovoltaics Specialists Conference, IEEE Conference Record, 1988, describes such a photovoltaic system, wherein in the case of the presence of various fault cases, the AC disconnecting element is actuated once a DC switching element has disconnected the inverter from the photovoltaic modules.
The article “Advanced, High-Reliability, System-Integrated 500-kW PV Inverter Development”, R. West, Final Subcontract Report NREL/SR-520-43839, 2008, likewise describes a photovoltaic system in which a DC switching element is arranged between photovoltaic modules and an inverter, and an AC disconnecting element is provided between a medium-voltage transformer and the energy supply grid. In the event of a disconnection operation of the inverter, for example in the event of an excessively low insolation intensity, opening both of the DC switching element and of the AC disconnecting element takes place.